This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-313387 filed on Oct. 27, 2005, the content of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor memory device which is configured to adjust the delay amount of an internal signal using data written into anti-fuses.
2. Description of the Related Art
Recent information processing apparatuses and the like are bottlenecked on access speeds to semiconductor memory devices, which limit system processing speeds, as a result of an increase in processing speeds performed by CPU. Thus, there is demand for a reduction in the time required to write and read data into and from semiconductor memory devices, and has been addressed by using DRAM (Dynamic Random Access Memory), SDRAM (Synchronous DRAM) and the like which operate at clock speeds at several hundred MHz. Further, developments are now in progress for DDR (Double Data Rate) 3-SDRAM which operates even at clock period tCK on the order of 1.25 to 2.5 ns.
In such semiconductor memory devices which operate at such high-speed clocks, a strict requirement is imposed on a Setup/Hold time (hereinafter called the “tIS/tIH”) of an internal signal to the clock. For example, DDR3-SDRAM is required to have a specified value of tIS/tIH in a range of 100-200 ps.
However, it is difficult to restrict tIS/tIH of all products within the specified range of values mentioned above when taking into account variations in manufacturing. As such, in order to improve the manufacturing yield rate, an adjusting operation is required for adjusting the delay amount of an internal signal to restrict tIS/tIH within a specified range of values (hereinafter called the “timing adjustment” in some cases).
Anti-fuses have been conventionally used for adjusting the timing of internal signals. The anti-fuse is a fuse element which records information (Fuse data) by producing an electrical breakdown therein. In semiconductor memory devices, Fuse data written into anti-fuses are read by a predetermined command signal, for example, upon power-on, and the delay amount of an internal signal is changed in accordance with the read Fuse data, thereby setting the tIS/tIH to fall within a specified range of values.
This adjustment of a delay time for an internal signal can be accomplished by supplying Fuse data AF[n:0] to a delay switching circuit, for example, as illustrated in FIG. 1, and controlling ON/OFF of a plurality of adjusting transistors, which make up the delay switching circuit, in response to Fuse data AF[n:0].
The delay switching circuit illustrated in FIG. 1 comprises a driver circuit made up of a P-channel MOS transistor and an N-channel MOS transistor which have their drains connected to each other; a plurality of adjusting P-channel MOS transistors connected in parallel for supplying a source voltage to the P-channel MOS transistor of the driver circuit, and a plurality of adjusting N-channel MOS transistors connected in parallel for supplying a ground potential to the N-channel MOS transistor of the driver circuit. Fuse data AF[n:0] read from an anti-fuse is supplied to the plurality of adjusting P-channel MOS transistors and the plurality of adjusting N-channel MOS transistors, connected in parallel, for use in controlling ON/OFF of the respective MOS transistors.
In such a circuit, an applied internal signal (CMDB signal) can be delayed by a desired amount before it is delivered in accordance with the number of adjusting P-channel MOS transistors and adjusting N-channel MOS transistors which operate together with the driver circuit.
For example, as reference data, n/2 Fuse data AF[*] are set to High (ON), and the remaining n/2 Fuse data AF[*] are set to Low (OFF)—as will be later described, each anti-fuse can be arbitrarily set to “0” or “1” before any data is written, and the set “0” or “1” can be inverted by writing appropriate data into the anti-fuse. Then, based on the reference data, when the internal signal (CMDB signal) is generated from an MRS CMD signal for initial setting delays in rising/falling, an adjusting P-channel MOS transistor (or adjusting N-channel MOS transistor) set to OFF is switched to ON by Fuse data to increase the driving ability to reduce the delay amount. On the other hand, when the internal signal (CMDB signal) is too early in rising/falling, a larger number of adjusting P-channel MOS transistors (or adjusting N-channel MOS transistors) are set to OFF to increase the delay amount.
Here, all the transistors that are described have the same size, but the number of anti-fuses can be reduced by changing the size of each transistor in proportion to the second power. By thus using the anti-fuses, tIS/tIH of the semiconductor memory device can be restricted within the specified range of values.
In this connection, anti-fuses are also used for remedying defective memory cells, in addition to timing adjustment for internal signals. In a semiconductor memory device, Fuse data written into anti-fuses when initially set are read to perform operations in accordance with the read Fuse data. The Fuse data for adjusting the timing of an internal signal are read by a Fuse read signal which is generated on the basis of an MRS CMD (Mode Register Set Command) supplied when an initial setting is made.
As illustrated in FIG. 2, a conventional anti-fuse control circuit comprises anti-fuse block 1, delay switching circuit 2, latch circuit 4, and Fuse read signal generator circuit 5.
Anti-fuse block 1 comprises a plurality of anti-fuses which record Fuse data, and supplies recorded Fuse data to delay switching circuit 2.
Delay switching circuit 2 switches the delay amount of an internal signal (CMDB signal) generated, for example, from the MRS CMD signal in accordance with the Fuse data supplied from anti-fuse block 1. Delay switching circuit 2 uses the circuit illustrated in FIG. 1.
Latch circuit 4 senses the CMDB signal generated from delay switching circuit 2 to generate a command (PMDCMDT) signal in synchronization with clock CLK.
Fuse read signal generator circuit 5 delivers the PMDCMDT signal supplied from latch circuit 4 as a Fuse read signal in accordance with a PMRS signal and an ADD2 signal.
When a delay amount of an internal signal is adjusted using anti-fuses, “0” is delivered when the anti-fuse is not written, while “1” is delivered when the anti-fuse is written on the assumption that the output signal of latch circuit 4 is delivered as is. On the other hand, if the output signal of latch circuit 4 is inverted before it is delivered, “1” is provided when the anti-fuse is not written, while “0” is provided when the anti-fuse is written, from which it is understood that the aforementioned adjusting P-channel MOS transistors and adjusting N-channel MOS transistors contained in delay switching circuit 2 can be arbitrarily set to ON/OFF.
The CMDB signal is generated from the MRS CMD signal which is generated in accordance with a variety of input signals of the semiconductor memory device when an initial setting is made. The PMRS signal and ADD2 signal are code signals which are generated in accordance with a variety of input signals of the semiconductor memory device when an initial setting is made for use as gate signals to deliver the Fuse read signal.
The Fuse read signal generated from Fuse read signal generator circuit 5 is supplied to anti-fuse block 1 which comprises a plurality of anti-fuses. Upon receipt of the Fuse read signal, anti-fuse block 1 delivers Fuse data that is recorded in the anti-fuses.
As represented in FIG. 3, latch circuit 4 senses the CMDB signal delivered from delay switching circuit 2, after the delay amount has been adjusted, and generates a command signal (PMDCMDT signal) required to read Fuse data from anti-fuse block 1. In this event, in order for latch circuit 4 to sense the CMDB signal, it is necessary to ensure a Setup time (tIS) which extends from the falling of the CMDB signal to the rising edge of clock CLK, and a Hold time (tIH) which extends from a rising edge of clock CLK to a rising edge of the CMDB signal, as mentioned above.
The PMDCMDT signal generated from latch circuit 4 is delivered as the Fuse read signal from Fuse read signal generator circuit 5 when the PMRS signal and ADD2 signals are both at “High.”
As illustrated in FIG. 4, anti-fuse block 1 comprises reset holder circuit 11 for holding the Fuse read signal, anti-fuse unit 12 for recording Fuse data, and fuse decode counter 13 for decoding information supplied from anti-fuse unit 12 to deliver Fuse data AF[n:0].
As illustrated in FIG. 5, anti-fuse block 1 is enabled to accept the Fuse read signal in response to an FFRST signal applied to reset holder circuit 11. Upon application of the Fuse read signal, reset holder circuit 11 holds the Fuse read signal, and generates an AFLOAD signal for supplying the Fuse data to anti-fuse unit 12.
Anti-fuse unit 12 comprises a plurality of storage elements, each of which is made up of a P-channel MOS transistor and N-channel MOS transistor which have their drains connected to each other with a fuse element interposed therebetween, where each storage element generates a “High” or a “Low” signal in accordance with the state (cut or uncut) of the fuse element. The signal delivered from each storage element is decoded by fuse decode counter 13 and then delivered as Fuse data AF[n:0].
Except in cases that involve different timing signal for delivering Fuse data AF[n:0], Anti-fuse block 1 may employ a circuit similar to a known anti-fuse block for use in remedying defective memory cells, and is not therefore limited to the circuit illustrated in FIG. 4.
For initial settings of semiconductor memory devices such as DRAM, semiconductor memory device has been conventionally operated in many cases by using a Power-On (hereinafter called “PON”) signal that is generated upon turning on the power or by a command (MRS CMD) signal when an initial setting is made. However, these signals are indefinite and therefore may sometimes cause a failure during the initial settings. For example, the PON signal may not be generated depending on a voltage value, its slope (Slew Rate) and the like when the semiconductor memory device is powered on by a system. Also, in a configuration where command signals are internally generated for initial settings, a shift in tIS/tIH of CLK and CMD, if any, can cause a failure to generate a command signal under strict tIS/tIH conditions at high operating frequencies. Stated conversely, a command signal can be always generated by adjusting the timing, but before the adjustment, there is no guarantee that the command signal will always be generated because how the delay amount of internal signal will be set when an initial setting is made is not known. Accordingly, future semiconductor memory devices, which are required to operate at higher frequencies, will encounter difficulties in ensuring secure operations because of these indefinite factors.
In the conventional anti-fuse control circuit described above, Fuse data are delivered from the anti-fuse block in response to the Fuse read signal generated when an initial setting is made. For this reason, if clock CLK and CMDB signal shift in phase relationship when an initial setting is made as shown in FIG. 6, this shift can cause tIS/tIH to fall out of the specified range of values, resulting in a failure to generate the PMDCMDT signal from the latch circuit and a consequent failure to generate the Fuse read signal.
In this event, since Fuse data are not delivered from the anti-fuse block, the delay amount of the CMDB signal is not switched in the delay amount switching circuit, thus impeding normal operation of the semiconductor device.
Also, a failure to generate Fuse read signal would disable an adjustment test for writing Fuse data into anti-fuses (the aforementioned adjustment of the timing for an internal signal), which is performed after assembly of the semiconductor memory device.
In future semiconductor memory devices, in particular, which are expected to operate at increasingly higher clock frequencies, increasingly more strict restrictions will be imposed on the aforementioned tIS/tIH, thus making it more difficult to guarantee normal operation of the semiconductor memory device due to the indefinite factors as mentioned above.